The present invention relates to aircraft propulsion systems and, more particularly, to a propulsion system embodying a variable cycle gas turbofan engine for use primarily in vertical or short takeoff and landing (V/STOL) aircraft.
In recent years, much attention has been directed toward developing what have been generally referred to as "variable cycle" engines, a family of hybrid gas turbofan engines which can operate with the high specific thrust, characteristic of a low bypass ratio turbofan or turbojet at supersonic speeds, and yet also exhibit the lower specific thrust, low noise and low fuel consumption, characteristic of a high bypass ratio turbofan. The need for such a variable cycle engine has arisen, in part, because of reduced world-wide fuel reserves, increased fuel costs and the expanded operational requirements projected for future aircraft. These factors tend to dictate that future engines will require reduced fuel consumption and increased operational flexibility. It is now well understood that aircraft which are to operate effectively and efficiently in both the subsonic and supersonic speed regimes will require such engines which exhibit the characteristics of both the high bypass ratio turbofan and the turbojet (or low bypass ratio turbofan). In addition, if the aircraft is projected to operate in the V/STOL mode (as from a mini-aircraft carrier or small airfield), large variations in propulsive thrust will also be required. For example, during takeoff and landing the engine must produce a very high level of lift thrust, a level in excess of the aircraft gross weight, and thus may be oversized for many of the forward flight cruise operating conditions. This resulting configuration of a grossly oversized cruise system is not easily adapted into a superior supersonic fighter aircraft. As of yet, no truly viable propulsion system has emerged which combines the necessary V/STOl lift features in an engine which offers effective mixed mission capabilities and reasonable life cycle cost projections.
Prior attempts at designing supersonic V/STOL fighters using state-of-the-art propulsive technology embodying powered lift have necessitated compromise in the propulsive cycle and engine design, resulting in aircraft having high takeoff gross weights or reduced operational capabilities. These prior art propulsion systems have either incorporated oversized engines (oversized with respect to the forward flight thrust requirements) to generate sufficient lift thrust or else incorporate separate lift engines which are utilized only in the powered lift mode. With either configuration, the resulting aircraft has an inordinately high takeoff gross weight which penalizes performance. Variable cycle engines offer a solution to this dilemma by providing a propulsion system which comprises a viable compromise for aircraft generally, and V/STOL aircraft in particular. Such a propulsive system must be able to produce very high thrust levels for powered lift during takeoff and landing and yet transition efficiently between the subsonic and supersonic operating modes. The present invention incorporates the novel approach to providing, efficiently, both the powered lift and cruise thrust of a V/STOL aircraft while not compromising the normal cruise operating modes, whether subsonic or supersonic.